Both of those are more expensive than pumped storage.

This depends on the time scale. For days or weeks? Pumped hydro is better. But for seasonal storage? The size of the reservoir becomes prohibitive.

A water reservoir is a heck of a lot cheaper to make large compared to a hydrogen tank.

Hydrogen on this scale would be stored underground, preferably in solution mined salt caverns. This is a well established technology, although it's more used for storing natural gas. These caverns are very inexpensive to build, which is why they are used.

Alberta, as it turns out, is underlaid by kilometers of sedimentary rock that includes salt formations.

Still sounds more expensive than a holding pond.

If one compares the heat of combustion of hydrogen stored at 100 bar and 20 C, to the energy of water elevated by 100 meters, the former exceeds the latter (per unit volume) by a factor of 1000. So that "holding pond" is going to be much, MUCH larger than the solution-mined cavern for storage systems of equal capacity (and remember one needs two reservoirs.)